Hearing aids with adaptive microphone matching systems that seek to balance long term characteristics of a pair of omni-directional microphones are known in the art. DE 198 22 021 to Siemens discloses a directional hearing aid with an adaptive analogue matching circuit which controls the gain of an adjustable preamplifier in an input signal channel. The value of the gain is derived from a measured difference in average output signal level between the input signal channels.
DE 198 49 739 to Siemens discloses a directional hearing aid that also comprises a pair of microphones and associated input signal channels. A DSP based adaptive matching algorithm is employed that allow characteristics of one of the input signal channels to be adjusted by a control element arranged in a feed-forward error correction loop. The error correction loop operates to determine a difference in average signal level between the pair of microphones and uses a detected difference to adjust a setting of the control element.
The above-mentioned hearing aids aim at compensating for long term drift in characteristics of the employed microphones and/or aim at making it feasible to use relatively low cost unmatched microphone pairs. However, there remains a need in the art for an adaptive matching methodology and hearing aid that allow for long time constants, preferably in the order of hours or days, for the adaptive matching process to avoid audible modulation of the microphone signal(s). The adaptive matching methodology employed should preferably also be well suited for implementation in a low power DSP of a digital hearing aid. Furthermore, the above-mentioned prior-art adaptive matching circuits and methods also lack means which are able to detect anomalous input signal conditions and either slow down or completely halt the adaptive matching process under such conditions e.g. by suitably steering the adjustment of a controlled element(s). Field trials and clinical research performed by the present inventors have demonstrated that an erroneous matching between the input signal channels is likely to occur if the adaptive matching process is allowed to continue, i.e. by adjusting the correction parameter value, under such anomalous input signal conditions.
Due to severe constraints on power consumption and size of hearing aid DSPs, it would furthermore be highly advantageous to design the adaptive matching circuit or algorithm in a way that minimises the use of DSP hardware and software resources, e.g. data word lengths and computational load, in particular multiplications.